Nebulization is extensively used in industry for many purposes such as paint spray systems and fuel burners. Humidifiers often use nebulization to increase the humidity in the air. Some types of analytical equipment use nebulizers to inject liquid samples into the measuring apparatus or heat source. This pneumatic nebulizer method was developed to enhance nebulizers for analytical equipment, but the method is applicable to any of the other uses of nebulizers. Analytical nebulizers require very precise, consistent, fine atomization. However, most present nebulizers have design methods which lead to plugging or salting with prolonged operation.
Pneumatic nebulizers all use the same essential principle (induction) to atomize the liquid: When gas at a higher pressure exits from a small hole (the orifice) into gas at a lower pressure, a gas jet is formed into the lower pressure zone, and the lower pressure gas is pushed away from the orifice. This creates a current in the lower pressure gas zone, and draws some of the lower pressure gas into the higher pressure gas jet. At the orifice, the draw of the lower pressure gas creates considerable suction, the extent depending on the differential pressures, the size of the orifice, and the shape of the orifice and surrounding apparatus. In all pneumatic nebulizers, the suction near the orifice is utilized to draw the liquid into the gas jet. The liquid is broken into small droplets in the process.
Present pneumatic nebulizer designs fit into four categories: 1. Concentric: Liquid flow surrounded by a gas flow or gas flow surrounded by a liquid flow; 2. Cross Flow: Gas flow at right angles to the Liquid flow; 3. Entrained: Gas and liquid mixed in the system and emitted as a combined flow; and 4. Babington types: Liquid is spread over a surface to decrease the surface tension, and passed over a gas orifice.
There are other non-pneumatic ways to atomize liquids, such as Ultra Sonic systems, and high pressure liquid injection, but they do not relate to the pneumatic nebulizer designs discussed in the present application.
1. Concentric Nebulizers
Concentric nebulizers have been in use for a long time. Some of the earliest patents awarded in Canada were for oil burner concentric nebulizers. Canadian Patent # 2405 of Apr. 18, 1873, as illustrated for example in prior art example 1 of FIG. 7A, is for a concentric nebulizer to enhance mixing steam and oil for a better burn in a furnace. The components are made of cast iron, and steel pipes, but the concept remains unchanged in analytical nebulizers such as the Meinhard brand of glass nebulizers sold for analytical equipment today, as illustrated for example in prior art example 2 of FIG. 7B, or the paint spray nozzle of Canadian Patent # 1013794 and 1014194 (1977) by the Black and Decker Company.
The concentric nebulizer works by the gas flow around the liquid orifice causing a suction on the liquid, drawing the liquid into the gas flow, mixing the liquid and gas, and spraying the mixture out in a generally uniform spray. The droplet sizes range considerably depending on the gas speed, volume, liquid viscosity, liquid surface tension, temperature, configuration, and other factors. Concentric nebulizer systems have the liquid and the gas passages narrow at the exit tip, to enhance the suction and to improve the mixing. They vary primarily in the liquid orifice position (either just inside the gas passage, even with the gas passage's end, or just extending past the gas passage) and in the presence of various means of blocking or shaping of the gas flow to enhance the mixing. However, if there are any particles in the liquid, they are most likely to plug the system at the tip since the tip has the smallest diameter. The Black and Decker patents # 1013794 & # 1014194 are primarily oriented towards production of a nozzle that is easy to disassemble and clean, since the common art was to throw nozzles away after each day's usage due to plugging.
Presently, the majority of all pneumatic nebulizer patents for any use are directed to concentric nebulizers.
2. Cross Flow Nebulizers
Cross flow nebulizers also have a long history and are commonly in use. More recent patents have not referred to the cross flow concept, but have rather referred to methods of assembling or providing the gas and liquid more efficiently. U.S. Pat. No. 4,344,574 is an example of a patent on a method for producing a cross flow nebulizer in an efficient and more accurate fashion, as illustrated in prior art example 3 of FIG. 7C. Canadian Patent # 2,044,712 refers to a method of providing atomization with a hand pumped gas source.
The cross flow nebulizer works by the gas flow across the liquid tip causing a suction on the liquid, drawing the liquid into the gas flow, mixing the liquid and gas, and spraying the mixture out in a generally uniform spray. The droplet sizes range considerably depending on the gas speed, volume, liquid viscosity, liquid surface tension, temperature, configuration, and other factors. Again, cross flow nebulizer systems have the liquid and the gas passages narrow at the tip to increase the suction and to improve the mixing. The liquid passage tip must be similar in diameter to the gas passage tip for the suction to be effective. Cross flow nebulizer systems vary primarily in the liquid tip position (either just below the gas passage, or slightly protruding into the gas passage's end). As with the concentric nebulizers, if there are any particles in the liquid, they are most likely to plug the system at the tip since the tip has the smallest diameter.
In general, cross flow nebulizers are neither as stable as the concentric nebulizers, nor as easy to build due to the critical alignment of the gas and liquid passages' tips.
3. Entrained Nebulizers.
For some liquids, nebulization can be improved by having the gas and liquid mix in an inner chamber and then emitted together from a single orifice. This technique has been applied to heavy liquids such as tar, as well as other liquids such as water. Canadian Patent # 1986 of Jan. 16, 1873, is for an entrained system, as illustrated for example in prior art example 4 of FIG. 7D. U.S. Pat. No. 4,284,239 of Aug. 18, 1981, is for an improved nebulizer for water, utilizing turbulent flow inside an entrained system to make tiny droplets, as illustrated for example in prior art example 5 of FIG. 7E.
The entrained nebulizers still rely on the gas and liquid being emitted from a small orifice. They differ from concentric and cross flow nebulizers in that the liquid and gas are mixed first, and then ejected from a small tip. The pressure within the entrained area helps to force the liquid and gas out the small tip, and assists the break up of the liquid into smaller drops. Note that one of the non-pneumatic types of nebulization is simply to force the liquid itself out of a small orifice. If forced out fast enough through a small enough orifice, the liquid will break up into small drops even without a gas stream being associated with it. As with the concentric and cross flow nebulizer systems, entrained nebulizer systems have the exit passages narrow at the tip. This still allows small particles to easily block the exit passage.
4. Babington Nebulizers.
In the late 1960s, Robert S. Babington and associates developed another form of nebulizer (Canadian Patent # 854061, U.S. Pat. Nos. 3,421,692; 3,421,699; 3,425,058; 3,864,326, U.S. Pat. No. 3,421,692 is illustrated for example in prior art example 6 of FIG. 7F). In this method, the liquid is introduced onto a smooth, unconfining surface having a gas orifice in the surface. The liquid forms a film on the surface, due to surface tension, or due to shape of the surface. This stresses the film of liquid before it reaches the gas orifice. The film of liquid passes over the gas orifice, and is further stressed by the passage of the gas out of the orifice. This causes minuscule particles of the liquid, estimated to be approx. 50 microns in size, to break away from the film and forms a fog like spray.
The Babington system works on many shapes of surfaces. Babington proposed a spherical surface with the liquid delivered to the top of the sphere, and the gas orifice at the side. Later adaptations include U.S. Pat. No. 4,206,160 of Jun. 3, 1980, and U.S. Pat. No. 4,880,164, of Nov. 14, 1989, as illustrated for example in prior art example 7 of FIG. 7G. They use a `V` shaped groove to direct the liquid towards the gas orifice, and use the sides of the `V` groove to provide the smooth surface upon which the liquid forms a film.
The most immediate advantage of the Babington system is that the liquid passage is not restricted in any portion of the path. Small particles do not plug the liquid passage, and no cleaning is necessary to maintain the flow as a result. Also, the thin film formed on the surface is readily broken into tiny drops, and produces excellent nebulization with very simple apparatus.
The main disadvantages to the Babington type systems are the requirements that the liquid must flow over the gas orifice due to gravitational forces, and that many materials have poor wetting abilities, so that the film becomes difficult or impossible to form. The usage of gravity to deliver the liquid requires that the nebulizer must be correctly oriented, or the liquid film may flow away from the gas orifice and no nebulization will occur. Some materials such as Teflon, are essentially non-wetting, and the liquid does not readily form a film. In working on designs similar to patent U.S. Pat. No. 4,880,164 with Teflon as the material used for the body of the nebulizer, the liquid is often found to flow away from the `V` groove. The non-wetting nature of the Teflon is stronger than the gravitational pull on the liquid.
It is apparent that all pneumatic nebulizers have the liquid broken into droplets by the induction action of a gas stream. The prior art nebulizers have various methods for delivering the liquid to the gas orifice, and some also use the suction at the gas orifice to draw the liquid up to the gas orifice. All have the above mentioned disadvantages, specifically being that they require specific orientations, or materials that are wettable, or have liquid passages that are narrowest at the orifices, leading to easy plugging.
There is only one essential requirement to produce atomization. This requirement is that the liquid must be brought close enough to the gas stream for the suction near the gas orifice to draw the liquid into the gas stream, and the liquid must be maintained at a level that does not cover the gas orifice. In a simple demonstration, one can provide a gas stream close to a body of water and the gas stream will atomize the water. For instance, as shown in FIG. 1, if a drinking cup of water is tilted so that the water is at the edge of the cup, and a gas stream is placed so that the gas orifice is just beside the water, the gas stream will draw the water to it, and produce a fine mist. FIGS. 2A-2C show the effects of the distance between the gas orifice and the liquid. If the gas orifice gets too close, the mist becomes a spray of water (FIG. 2C). If the gas orifice is too far away, the mist stops (FIG. 2A). Here the appropriate distance is on the order of 0.5 mm for a good mist (FIG. 2B).
In this simple example, several points are demonstrated: The water in the cup is a very large body of water compared to the gas orifice. The water arrived without any constraints on the path. There is no thin film required to decrease the surface tension as in the Babington method. The amount of liquid atomized is determined by the gas orifice size and pressure differential, not by the amount of liquid available. The liquid is drawn to the gas orifice by the suction at the orifice. As the liquid is atomized, the surface tension of the liquid will form a path to the gas orifice and maintain the flow of the water to the gas stream until there is a large change in the distance of the liquid from the gas orifice.
In applying this procedure to a device that can be manufactured, several methods may be used. The methods will work as long as the liquid surface can be maintained close enough to the gas orifice so that the surface tension of the liquid can maintain a path to the gas orifice. For instance, a stream of water of any diameter that is maintained in a smooth flow close to the gas orifice will act as an appropriate source to allow atomization of the liquid. The stream need not be constrained in a passage.
In normal usage of nebulizers, it is necessary to constrain the liquid in a passage to maximize the device's control of the process, and to minimize the liquid required to be delivered to the gas orifice. It is desirable for most applications that all of the liquid delivered to the gas orifice should be atomized. Many methods that allow the liquid to be maintained at an appropriate distance from the gas orifice, while enabling those skilled in the art to manufacture such a device with minimal effort will be apparent to those skilled in the art.
The method and apparatus of the present invention utilizes the surface tension of liquid, along with the natural induction caused by a gas stream out of an orifice to produce atomization of the liquid.
The nebulizer and method for dispersing liquids into a gas of the present invention provides improvements to the above-described problems in the conventional systems and methods and the nebulizer apparatus and method of the present invention form a new design category as will be explained later.